The overall objectives of this project are threefold. First, characterization of the development of brown adipose tissue (BAT) metabolism in the fetus and newborn will be continued. Second, the mechanism whereby thyroid hormones influence BAT function and metabolism in the fetus and newborn will be clarified, and finally, the role that BAT plays in diet induced thermogenesis (DIT) in the newborn will be evaluated. Our specific aims are: 1) To characterize the maturation of BAT mitochondrial purine nucleotide binding, catecholamine receptors, and adenyl cyclase activity in fetal and newborn rabbits. 2) To determine the effect of changes in thyroid status on BAT cellular respiration, lipolysis, lipogenesis, mithochondrial purine nucleotide binding, catecholamine receptors and adenyl cyclase activity in the newborn rabbit. 3) To determine the effect of a single meal and the chronic ingestion of a high fat or carbohydrate diet on in vivo oxygen consumption, BAT temperature, BAT composition, BAT cellular respiration, lipolysis, lipogenesis, mitochondrial purine nucleotide binding, catecholamine receptors and adenyl cyclase activity in the newborn rabbit. 4) To determine the effect of the chronic ingestion of a high fat or carbohydrate diet on body composition and growth in the newborn rabbit. 5) To determine if BAT mediated DIT can be manipulated by either diet, feeding frequency or blocking agents in the newborn rabbit. The methods used will include the measurement of in vivo oxygen consumption, BAT temperature, in vitro cellular respiration, lipolysis, lipogenesis, catecholamine receptors, cellular enzymes, thyroid hormones by RIA, and body composition. These studies will increase our understanding of neonatal thermogenesis and possibly lead to new therapeutic approaches that will minimize energy expenditure for thermogenesis and maximize energy utilization for growth in both premature and full term infants.